In most vertebrate neurons, action potentials are triggered at the distal end of the axon initial segment (AIS). Both position and length of the AIS vary across and within neuron types, with activity, development and pathology. What is the impact of AIS geometry on excitability? Direct empirical assessment has proven difficult because of the many potential confounding factors. Here we carried a principled theoretical analysis to answer this question. We provide a simple formula relating AIS geometry and sodium conductance density to the somatic voltage threshold. A distal shift of the AIS normally produces a (modest) increase in excitability, but we explain how this pattern can reverse if a hyperpolarizing current is present at the AIS, due to resistive coupling with the soma. This work provides a theoretical tool to assess the significance of structural AIS plasticity for electrical function.

中文翻译：

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轴突初始节段几何形状与兴奋性之间的理论关系

在大多数脊椎动物神经元中，动作电位是在轴突初始节段（AIS）的远端触发的。AIS的位置和长度在神经元类型之间和内部随活动，发育和病理而变化。AIS几何形状对兴奋性有何影响？由于许多潜在的混杂因素，直接的经验评估已被证明是困难的。在这里，我们进行了有原则的理论分析来回答这个问题。我们提供了一个简单的公式，将AIS几何形状和钠电导密度与躯体电压阈值相关联。AIS向远侧移动通常会引起（适度）兴奋性增加，但是我们解释了如果由于与体之间的电阻耦合而在AIS处存在超极化电流，则该模式如何可以反向。